Manganese dioxide is well known as an active material for a cathode, for example, for a manganese dry battery, especially for an alkaline manganese dry battery. Manganese oxide exhibits good durability and is inexpensive.
An alkaline manganese dry battery provided with a cathode comprising electrolyte manganese dioxide as an active material has excellent discharge characteristics at a heavy-load discharge. Therefore, the alkaline manganese dry battery is widely used for, for example, electronic cameras, portable tape recorders, personal digital assistants, game machines and toys, and the demand therefor is rapidly expanding in recent years. However, the alkaline manganese dry battery has a problem such that, with an increase of discharge electric current, the utilization of manganese dioxide as an active material for a cathode decreases, and the discharge voltage is reduced with the result of large reduction of discharge capacity. In other words, in the case when an alkaline manganese dry battery is equipped in an instrument utilizing a large electric current within a short period of time, i.e., a high-rate discharge is conducted, the utilization of manganese dioxide packed as a cathode active material is reduced and the durability of the instrument decreases. Therefore, manganese dioxide having improved high-rate characteristics, namely, capable of exhibiting a high capacity and an enhanced durability under intermittent high-rate discharge conditions in which a large electric current is taken in a short period of time is eagerly desired.
As manganese dioxide for use in a cathode active material for an intermittent high-rate discharge, manganese dioxide having enhanced high-rate characteristics was proposed in Japanese Unexamined Patent Publication No. 2002-304990, which has a surface sulfate content of at least 0.10% by weight, and a controlled alkali metal content of smaller than 0.20% by weight. Further, electrolytic manganese dioxide having a sulfate group content in the range of 1.3% to 1.6% by weight was proposed in Japanese Unexamined Patent Publication No. 2004-047445 (U.S. Pat. No. 3,712,259). Although the high-rate characteristics of the manganese dioxide proposed in these patent publications are enhanced to some extent, the extent of enhancement is not sufficiently high. Further, with the electrolyte manganese dioxide containing a sulfate group, problems of reduction of durability of a dry battery and unstable voltage thereof often occur.
Improved manganese dioxide having a controlled sodium content X and a controlled JIS-pH value Y, which satisfy the following formula: Y>4.7X+2.4, and which manganese dioxide specifically has a JIS-pH value of 3.5 to 5.0, was proposed in Japanese Unexamined Patent Publication No. 2001-026425. Although the high-rate characteristics of the manganese dioxide proposed in this patent publication is enhanced to some extent, the extent of enhancement is still not sufficiently high, Further, a problem of corrosion often occurs in the process for making the battery.